1. Field of the Invention
The present invention relates to a semiconductor device including a resistor made of polycrystalline silicon.
2. Description of the Related Art
In a semiconductor integrated circuit, a diffused resistor or a polycrystalline silicon resistor is used. The diffused resistor is made from a single crystalline silicon semiconductor substrate into which impurities of an opposite conductivity type to that of the semiconductor substrate are implanted. The polycrystalline silicon resistor is made of polycrystalline silicon into which impurities are implanted. In particular, advantages in the polycrystalline silicon resistor such as small leakage current brought by insulating films surrounding the resistor and high resistivity obtained by defects existing at grain boundaries result in wide use in semiconductor integrated circuits.
FIGS. 2A and 2B are a schematic plan view and a schematic sectional view of a conventional polycrystalline silicon resistor circuit, respectively. The polycrystalline silicon resistor is produced by implanting a p-type or n-type impurity to a polycrystalline silicon thin film deposited on an insulating film by LPCVD or the like, and then processing the resultant into a resistor shape with a photolithography technique. Impurity implantation is performed for determining a resistivity of the polycrystalline silicon resistor. Depending on a desired resistivity, a concentration of the p-type or n-type impurity to be implanted ranges from 1×1017/cm3 to 1×1020/cm3. Further, at each terminal on both sides of the resistor, a contact hole and a metal wiring are formed to get the potential thereof. A satisfactory ohmic contact between the polycrystalline silicon and the metal wiring at the terminal requires selective implantation of impurities at a high concentration of equal to or more than 1×1020/cm3 are, by using a patterned photo resist, into a part of the polycrystalline silicon corresponding to the terminal of the resistor. The resistor using the polycrystalline silicon is thus structured, as shown in the schematic plan view of FIG. 2A and the schematic sectional view of FIG. 2B, the resistor includes a polycrystalline silicon 103 made of a low concentration impurity region 104 and a high concentration impurity region 105, which is formed on a insulating film 102 on a semiconductor substrate 101. Potential is obtained from a metal wiring 107 via a contact hole 106 disposed on the high concentration impurity region 105.
In order to obtain various potentials from a resistor circuit, various resistor groups are structured by connecting basic unit resistors in series or in parallel and provided with terminals for supplying the potentials. In order to stabilize a resistance for each resistor group, a metal portion is formed on the resistor group and connected to a terminal at an end of the resistor group. This structure is illustrated in FIG. 2B and there are two reasons for the structure.
The first reason is to obtain stability of the polycrystalline silicon resistor. Since polycrystalline silicon is a semiconductor, formation of a wiring or an electrode over the polycrystalline silicon causes depletion or accumulation in the polycrystalline silicon owing to a relative relationship between the potential of the wiring or the electrode and that of the polycrystalline silicon resistor, which varies a resistance of the polycrystalline silicon resistor. To be specific, existence of a wiring or an electrode having a higher potential than the polycrystalline silicon resistor made of the polycrystalline silicon into which the p-type impurity is implanted, and disposed directly above the polycrystalline silicon depletes the p-type polycrystalline silicon, which increases the resistance of the resistor. In a case of a reverse relationship in potential, the resistance reduces owing to the accumulation. Intentional formation of a wiring having a potential close to that of the polycrystalline silicon on the polycrystalline silicon enables the maintenance of a constant resistance, thus preventing the variation of the resistance. This is illustrated in the plan view of FIG. 2A as an example. In FIG. 2A, an electrode at one side of the polycrystalline silicon resistor is extended to a resistor to fix the potential. This phenomenon depends not only on the wiring above the polycrystalline silicon but of course on the wiring below the polycrystalline silicon. In other words, a relative relationship between potentials of the polycrystalline silicon resistor and a semiconductor substrate located below the polycrystalline silicon resistor varies the resistance. In view of this, there is known a method of stabilizing the potential by intentionally forming a diffusion region (not shown) or the like below the polycrystalline silicon resistor in the same manner as the above-mentioned metal wiring.
The second reason is to prevent hydrogen, which affects the resistance of the polycrystalline silicon, from diffusing into the polycrystalline silicon. The polycrystalline silicon is composed of a grain having relatively high crystallinity and a grain boundary between the grains which has low crystallinity, that is, a high interface level density. The resistance of the polycrystalline silicon resistor is mostly determined by electrons or holes, which serve as carriers, trapped by a large number of interface levels existing at the grain boundary. When hydrogen having a high diffusion coefficient is, however, generated in a semiconductor manufacturing process, the generated hydrogen easily reaches the polycrystalline silicon and becomes trapped by the interface level, thus varying the resistance. Examples of the hydrogen generating process include a sintering step in a hydrogen atmosphere after metal electrode formation and a forming step of a plasma nitride film using an ammonia gas. When the metal portion overlaps the polycrystalline silicon resistor, the variation of the resistance of the polycrystalline silicon due to the hydrogen diffusion can be suppressed.
The method of stabilizing the resistance of the polycrystalline silicon is disclosed in, for example, JP 2002-076281 A.
The method of stabilizing the resistance of the polycrystalline silicon has the following problem. That is, the metal portion on the polycrystalline silicon is susceptible to factors other than hydrogen which affect the polycrystalline silicon in the semiconductor manufacturing process, which includes heat, stress, or charging due to plasma. Therefore, those factors affect the polycrystalline silicon through the metal portion thereon, which results in the variation of the resistance. This will be explained in detail below with reference to FIG. 2A.
It is assumed that the unit resistor constituting the resistor formed with the manufacturing method mentioned in Description of the Related Art section is formed to have a constant area. In this case, when the areas of the metal portions overlapping the resistors are the same like resistor groups 1, 2, and 3 of FIG. 2A, a resistance ratio is obtained with preferable precision. However, when the metal portion is provided so as to overlap a plurality of resistors like a resistor group 4 or 5 of FIG. 2A, it is found that resistance ratio accuracy of the resistors is not constant. For example, since the resistor group 1 includes one unit resistor, and the resistor group 4 includes two unit resistors connected in series, the resistance ratio of 1:2 should accordingly be obtained. However, the desired resistance ratio can not obtained in many cases and the resistance ratio of, for example, 1:1.7 is obtained.